Comparative study on the mechanical and microstructural characterisation of AA 7075 nano and hybrid nanocomposites produced by stir and squeeze casting

In this research work, a comparative evaluation on the mechanical and microstructural characteristics of aluminium based single and hybrid reinforced nanocomposites was carried out. The manufacture of a single reinforced nanocomposite was conducted with the distribution of 2 wt.% nano alumina particles (avg. particle size 30–50 nm) in the molten aluminium alloy of grade AA 7075; while the hybrid reinforced nanocomposites were produced with of 4 wt.% silicon carbide (avg. particle size 5–10 mm) and 2 wt.%, 4 wt.% nano alumina particles. Three numbers of single reinforced nanocomposites were manufactured through stir casting with reinforcements preheated to different temperatures viz. 400 C, 500 C, and 600 C. The stir cast procedure was extended to fabricate two hybrid reinforced nanocomposites with reinforcements preheated to 500 C prior to their inclusion.

Original Article

Comparative study on the mechanical and microstructural

characterisation of AA 7075 nano and hybrid nanocomposites produced

by stir and squeeze casting

School of Mechanical Engineering, VIT University, Vellore 632014, India

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:

Received 5 December 2016

Revised 13 February 2017

Accepted 28 February 2017

Available online 8 March 2017

Keywords:

Hybrid nanocomposites

AA 7075

Alumina

a b s t r a c t

In this research work, a comparative evaluation on the mechanical and microstructural characteristics of aluminium based single and hybrid reinforced nanocomposites was carried out The manufacture of a single reinforced nanocomposite was conducted with the distribution of 2 wt.% nano alumina particles (avg particle size 30–50 nm) in the molten aluminium alloy of grade AA 7075; while the hybrid rein-forced nanocomposites were produced with of 4 wt.% silicon carbide (avg particle size 5–10mm) and

2 wt.%, 4 wt.% nano alumina particles Three numbers of single reinforced nanocomposites were manu-factured through stir casting with reinforcements preheated to different temperatures viz 400°C,

500°C, and 600 °C The stir cast procedure was extended to fabricate two hybrid reinforced nanocompos-ites with reinforcements preheated to 500°C prior to their inclusion A single reinforced nanocomposite

http://dx.doi.org/10.1016/j.jare.2017.02.005

2090-1232/Ó 2017 Production and hosting by Elsevier B.V on behalf of Cairo University.

Peer review under responsibility of Cairo University.

⇑ Corresponding author.

E-mail address: kannan.chidambaram@vit.ac.in (C Kannan).

Contents lists available atScienceDirect

Journal of Advanced Research

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j a r e

Silicon carbide

Squeeze casting

Stir casting

was also developed by squeeze casting with a pressure of 101 MPa Mechanical and physical properties such as density, hardness, ultimate tensile strength, and impact strength were evaluated on all the devel-oped composites The microstructural observation was carried out using optical and scanning electron microscopy On comparison with base alloy, an improvement of 63.7% and 81.1% in brinell hardness was observed for single and hybrid reinforced nanocomposites respectively About 16% higher ultimate tensile strength was noticed with the squeeze cast single reinforced nanocomposite over the stir cast

Ó 2017 Production and hosting by Elsevier B.V on behalf of Cairo University This is an open access article

under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)

Introduction

Aluminium metal matrix composite (AMMC) is being preferred

for numerous engineering applications like aerospace, marine,

automobile and mineral processing due to their lightness

associ-ated with remarkable specific strength and thermal properties

[1–5] In aluminium composites, the properties like high toughness

and ductility associated with aluminium matrix are combined with

superior properties of ceramics such as high strength and elastic

modulus by adding ceramic reinforcements in the base matrix

[6,7] Alumina (Al2O3), silicon carbide (SiC) and graphite (Gr) are

the most common reinforcing materials[8,9]which can be

incor-porated in the base aluminium matrix in the form of whiskers or

particles However, manufacturing complexity and low cost favour

the particle reinforced composite over whisker-reinforced[10,11]

Metal matrix nano composites (MMnC) are a new category of

materials, in which the reinforcements in the range of

nano-meter size are being used[12] Increased surface area offered by

nano scale reinforcements at the matrix interface leads to superior

properties in composites such as increased mechanical strength,

higher fatigue life and better creep resistance at elevated

temper-ature without much compromise on ductile characteristics [13–

15] However, the end properties of MMnCs are greatly influenced

by the size, shape, uniform distribution, hardening mechanism and

thermal stability of nano reinforcements [16,17] Hybrid metal

matrix composite (HMMC) is being investigated by various

researchers around the world due to their enhanced properties

over single reinforced composites These composites are formed

either by a combination of two or more reinforcements in different

forms like particulates, whiskers, fibres and nanotubes or two

dif-ferent reinforcements of the same form The primary and

sec-ondary reinforcements can be blended in a way to optimise the

properties of hybrid composites Improved mechanical properties

were observed with hybrid reinforced nanocomposites over single

reinforced nanocomposites due to a significant reduction in

menis-cus penetration defect and inter-metallic component formation

[18–23]

At present, the vehicle manufacturers are trying various

meth-ods to enhance the efficiency This necessitates the automobile

components to be manufactured from lightweight materials

Across the globe, the researchers are putting their efforts to

develop light materials in the form of composites for aerospace

and automobile applications[24,25] Despite their efforts, limited

research is available on hybrid reinforced nanocomposites that

are based on aluminium alloy AA 7075, which has zinc as a primary

alloying element It has excellent strength to weight ratio The

fati-gue strength of this material is comparatively better than many

other aluminium alloys[26] The limited exploration on AA 7075

hybrid reinforced nanocomposite demands further investigation

Hence, in this investigation, single and hybrid reinforced

nanocom-posites were manufactured with the incorporation of nano alu-mina and micro silicon carbide particles as reinforcements in base matrix of AA 7075 High hardness, excellent stability and bet-ter insulation are the most inbet-teresting properties of Al2O3 [27];

Table 1

Chemical composition of AA 7075 [8,9]

(a) UV- Vis spectrometer reading

(b) TEM analysis

Fig 1 Spectrometric and TEM analysis of nano Al 2 O 3 particles.

Table 2 Properties of reinforcements [26,27]

Density (g/cm 3

Coefficient of thermal expansion (
10 6 /°C) 8.4 4.3

while SiC has excellent oxidation resistance up to 1650°C and

thermal shock resistance High thermal conductivity, low thermal

expansion and high strength of silicon carbide are imputed to these characteristics [28] Al2O3 nanoparticles preheated to different temperatures were added to molten metal This was performed

to examine the influence of particle preheat temperature on its uniform distribution In addition, a single reinforced

nanocompos-Fig 2 Bottom type stir casting set up with squeeze casting attachment.

Table 3

Design of Experiments (DOE) for the fabrication of single reinforced nanocomposites.

Type of composite Preheating temperature of reinforcements (°C) Squeezing pressure (MPa)

Low level (1) Medium level (0) High level (+1) Low level (1) High level (+1)

Table 4

Processing methods of single and hybrid reinforced nanocomposites.

Sample Description Description of

pre-processing

Processing method

2 AA 7075 reinforced with 2 wt.

% nano Al 2 O 3 particles

Al 2 O 3

nanoparticles preheated to

400 °C

3 AA 7075 reinforced with 2 wt.

% nano Al 2 O 3 particles

Al 2 O 3

nanoparticles preheated to

500 °C

4 AA 7075 reinforced with 2 wt.

% nano Al 2 O 3 particles

Al 2 O 3

nanoparticles preheated to

600 °C

5 AA 7075 reinforced with 2 wt.

% nano Al 2 O 3 particles and

4 wt.% micro SiC particles

Both Al 2 O 3 and SiC particles preheated to

500 °C

6 AA 7075 reinforced with 4 wt.

% nano Al 2 O 3 particles and

4 wt.% micro SiC particles

7 AA 7075 reinforced with 2 wt.

% nano Al 2 O 3 particles

Al 2 O 3

nanoparticles preheated to

500 °C

Squeeze casting pressure of

101 MPa

Fig 3 Theoretical and experimental densities of single and hybrid reinforced nanocomposites.

ite was manufactured with squeeze casting to analyse the effect of

squeezing pressure on the improvement of mechanical properties

over a stir cast nanocomposite The reinforcement inclusion in

the molten metal and stirring for a prescribed time was followed

by transferring molten metal into a die steel mould of squeeze

casting arrangement by opening the furnace valve using automatic

control This was immediately followed by the application of

squeezing pressure over the solidifying composite metal The die

set up was cooled by water circulation that enhances the final

mechanical properties of composites through cooling effect The

solidified nanocomposite taken out of die setup was of /50 mm

diameter and 300 mm length All hybrid reinforced

nanocompos-ites considered in this investigation were produced through stir casting A comparative evaluation was performed on the mechan-ical properties of single and hybrid reinforced nanocomposites pro-duced through different processing techniques (preheating of reinforcements prior to their inclusion in the matrix, stir casting, and squeeze casting) and presented in this work

Material and methods Aluminium alloy of grade AA 7075 was selected as the base matrix and it was melted in the resistant heating furnace that has an integral stirrer Nano size (30–50 nm) Al2O3and micron size (5–10mm) SiC particles were added as reinforcements for the cur-rent investigation The chemical composition of AA 7075 and the properties of reinforcing materials are listed inTables 1 and 2,

respectively The UV–visible spectrometric and transmission elec-tron microscopic (TEM) analysis of nano Al2O3particles is shown

in Fig 1(a) and (b) respectively The absorbance of light, while passing through a sample was measured using UV–visible double beam spectrophotometer (Hitachi model U-2800) in the spectrum

of 380–600 nm In the spectrometric analysis, the absorbed light peak at a wavelength of 400 nm showed the presence of alumina particles The size and morphology of nano alumina particles were determined using TEM (Philips CM12 Transmission Electron Microscope, Netherlands) About 10 mg/L of Al2O3 nanoparticles was plunged in acetone solution which was preceded by 5 min ultrasonic treatment The dispersed particles were then deposited onto the lacey-carbon-coated copper grid Nearly spherical parti-cles of 30–50 nm were observed from the micrograph During the melting of base matrix, about 10 g of sodium aluminium hexafluo-ride (Na3AlF6) was added to the melt to prevent slag formation and

to improve the efficiency of the casting process This was done

Fig 4 Porosity of single and hybrid reinforced nanocomposites.

Fig 5 Hardness of AA 7075, single and hybrid reinforced nanocomposites (as cast

condition).

Fig 6b True stress–true strain curves for single reinforced and hybrid reinforced nanocomposites.

prior to the addition of reinforcements in the molten metal

Rein-forcement preheating was attempted by several researchers[29–

31]to remove surface impurities, alter the surface composition

and for desorbing the gases Previous research works performed

with other aluminium alloys considered the reinforcement

pre-heating temperatures in the range of 200–800°C Based on the

consideration of existing literature and the capacity of available

equipment, the reinforcement preheating temperatures of 400,

500 and 600°C are going to be adopted in this work Regardless

of the base matrix, several weight proportions of the reinforcement

(0–3.5 wt.%) were being tried by previous researchers

[32,33,15,34] Existing literature revealed that uniform distribution

of nano reinforcements in the melt could be achieved, keeping

their weight fraction not exceeding 2% In most cases, a declining

trend in the mechanical properties was observed, when this weight

fraction exceeded Thus, three single reinforced nanocomposites

were produced by stir casting with the inclusion of 2 wt.% nano

Al2O3 particles, which were preheated to 400°C, 500 °C and

600°C prior to their inclusion This was done to investigate the influence of reinforcement preheat temperature on the mechanical characteristics of composites, thus produced Another single rein-forced nanocomposite was produced with squeeze casting to per-form the quantitative comparison of mechanical characteristics with that of stir cast composite The hybrid reinforced nanocom-posites could be developed keeping the weight fraction of sec-ondary reinforcement either one-half or same as that of primary reinforcement to investigate the influence of secondary reinforce-ment on the end properties of the composites Thus, two hybrid reinforced nanocomposites were developed through stir casting with the incorporation of 2 wt.% and 4 wt.% nano Al2O3 with

4 wt.% micro SiC particles in the melt Based on preliminary studies accomplished on stir cast composites, optimised processing parameters such as stirrer speed of 600 rpm, reinforcement flow rate of 5 g/min and stirring time of 4 min were adopted for the fab-rication of all composites[30,35]

Fabrication procedure The bottom type stir casting set up used for manufacturing of single and hybrid reinforced nanocomposites is shown in Fig 2 About 1.2 kg of AA 7075 was melted in a graphite crucible, which was heated to a temperature of 850°C When the melt tempera-ture was about 30°C above the pouring temperature, the pre-heated stirrer was introduced in the melt The stirrer was made

to run at 600 rpm for two minutes While the agitation is being continued, the preheated reinforcement or mixture of reinforce-ments was introduced into the melt Al2O3 reinforcement was maintained at 2 wt.% in single reinforced nanocomposite, while it was varied as 2 wt.% and 4 wt.% for hybrid reinforced nanocompos-ites The secondary reinforcement in these hybrid nanocomposites

is 4 wt.% SiC The stirring was continued for another 4 min to ensure the proper mixing of the matrix and the reinforcement The molten metal was then poured into the preheated steel moulds

to obtain the castings The adopted design of experiments (DOE) for the fabrication of single reinforced nanocomposites is pre-sented inTable 3

Test specimens were fabricated from these castings using a wire-cut electro discharge machine (WEDM) The notation for the test samples and description of their processing methods are listed

in Table 4 Mechanical characterisation tests such as hardness, porosity, tensile strength, impact strength and microstructural evaluation were performed on these test specimens Whilst Archi-medes principle was adopted to measure the experimental

den-Fig 6c Tensile strength and ductility variation of single and hybrid reinforced

nanocomposites.

Fig 7a Schematic and photographs of impact testing samples (All dimensions in

mm).

Fig 7b Impact strength of single and hybrid reinforced nanocomposites.

sity; the tensile strength of the composites was determined using

the universal testing machine Optical Brinell hardness testing

machine was used to observe the hardness The microstructure

and distribution of reinforcements in the base matrix were

exam-ined using an optical microscope and scanning electron

microscope

Results and discussion

Density and porosity

The theoretical and experimental density of single and hybrid

reinforced nanocomposites under investigation are shown in

Fig 3 The theoretical density of a nanocomposite was determined

using the rule of mixtures and can be represented as

qtheoretical¼qmumþqrur ð1Þ

whereum andur represent wt fraction of matrix and

reinforce-ment;qm andqr represent density of matrix and reinforcement;

q represents the theoretical density of a composite The rule

of mixtures was adopted to compute the theoretical density of a nanocomposite; whilst Archimedes principle was employed to determine the experimental density[21,36–38] Nano Al2O3 and micro SiC particles used as reinforcements in this investigation have density values of 3970 kg/m3and 3210 kg/m3respectively Due to the higher density of these reinforcements over the base matrix, the theoretical density of a nanocomposite was found to increase

in proportion with wt.% of reinforcements The experimental den-sity of all developed single and hybrid reinforced nanocomposites was found to follow the trend of theoretical density, which indi-cated the successful fabrication of these composites through stir and squeeze casting The hybrid nanocomposite reinforced with

4 wt.% nano Al2O3and 4 wt.% SiC was found to have the highest density among all samples This might be imputed with high den-sity Al2O3particles For the same level of nanoparticle reinforce-ment (2 wt.%), the squeeze casting results in much higher density over the stir casting, which can be clearly inferred from sample 7

inFig 3 The procedure of determining the theoretical and experimental density of a composite through the respective utilisation of rule of mixtures and Archimedes principle was subsequently followed by Fig 8 Optical micrographs of aluminium alloy (a), stir cast single reinforced nano composites (b–d), stir cast hybrid reinforced nanocomposites (e and f) and squeeze-cast single reinforced nanocomposite (g).

porosity computations It was found that porosity of both single

and hybrid reinforced nanocomposites was higher than

unrein-forced alloy This might have been associated with issues such as

poor wettability characteristics, particle agglomeration, clustering

and pore nucleation at the interface with inadequate mechanical

stirring [39,40] Generally, the agglomeration of reinforcement

and subsequent clustering provides a hindrance to the liquid metal

flow The preheating of reinforcement could reduce the wettability

issues imposed by nanoparticles and lead to better distribution in

the molten metal[41] The influence of preheating temperature

(400°C, 500 °C and 600 °C) and effect of squeezing pressure on

the percentage porosity of nanocomposites was studied The

per-centage porosity was calculated for all composites using the

relation

% porosity ¼Theoretical density Experimental density

ð2Þ

The porosity in the metal matrix composites is instituted due

to the improper interfacial reaction between the ceramic rein-forcements and the matrix This interfacial reaction is principally influenced by the factors such as free energy at the interface, con-vection properties and temperature gradient that exists between particles and matrix during solidification in addition to other parameters viz stirring speed, melt viscosity, clustering, the den-sity difference between melt and particles[42] With an invariable reinforcement (2 wt.%) in single reinforced nanocomposites, the particles preheated at 500°C was found to result in low porosity

Fig 9 SEM images of base aluminium alloy (a), stir-cast 2 wt.% Al 2 O 3 nanocomposite (c), stir-cast hybrid nanocomposite with 2 wt.% Al 2 O 3 and 4 wt.% SiC (e), stir-cast hybrid nanocomposite with 4 wt.% Al 2 O 3 and 4 wt.% SiC (g) squeeze cast 2 wt.% Al 2 O 3 nanocomposite (h) EDS of aluminium alloy (b), nano Al 2 O 3 particles (d) SiC particles (f)

over the other preheating temperatures, 400°C and 600 °C This

might have been resulted due to the favourable convection

prop-erties and temperature gradient that established with the particle

preheated temperature of 500°C The porosity of this single

rein-forced nanocomposite was further scaled down to 0.7% through

squeeze casting This is due to the fact that the plastic working

induces the pore closing [43] The calculated porosity of single

and hybrid reinforced nanocomposites is shown in Fig 4 An

appreciable amount of porosity (4.6%) was observed with a

hybrid reinforced composite, which possessed 4 wt.% nano Al2O3

and 4 wt.% micro SiC particles The calculations revealed that

hybrid reinforced nanocomposites were found to possess higher

porosity when compared to single reinforced nanocomposites

Increased weight fraction of nanoparticle raises the ratio of

agglomeration that might have resulted in this appreciable

increase in porosity; which can be reduced through squeeze

casting

Brinell hardness The hardness of single and hybrid reinforced nanocomposites was determined according to ASTM E10-07 at room temperature

of 30°C Brinell hardness tester with a 10 mm ball indenter and

500 kg was used for 30 s The measurements were taken at five different locations on each sample to acquire an average hardness value The hardness variation for different composite samples is shown in Fig 5 Increased hardness values were observed with

an increase in weight percentage of nano Al2O3 particles Maximum hardness was observed with sample 3 (preheated nanoparticles 500 °C) amidst the single reinforced stir cast nanocomposites This is due to the uniform distribution of nanoparticles in the base matrix In the case of hybrid reinforced nanocomposites, higher hardness was observed with sample 5 In spite of increased weight fraction of nano alumina particles, sam-ple 6 was found to possess lower hardness than samsam-ple 5, which Fig 9 (continued)

might be due to agglomeration of nanoparticles (Fig 9d) Among

all the investigated composites, the squeeze cast nanocomposite

(sample 7) that composed of 2 wt.% nano Al2O3particles was found

to possess the highest hardness This might be attributed to lowest

porosity and extreme grain refinement in the case of squeeze cast

nanocomposite over other composites In general, when the cast

composites are cooled to the room temperature, the ceramic

rein-forcements viz nano Al2O3and micro SiC particles considered in

this investigation tend to strengthen the matrix due to their

mis-match in thermal expansion coefficient (CTE) of the alloy matrix

This, in turn, induced the mismatch strains at the interfaces of

rein-forced nanoparticles and matrix which hinder the dislocation

movement and resulting in improved hardness of the composites

Higher hardness was observed with the hybrid reinforced

nanocomposites due to stronger Al2O3-SiC interface in comparison

to Al–Al2O3interface in the case of single reinforced

nanocompos-ite, which can be inferred fromFig 5 In comparison with the base

alloy, an improvement of about 63.7% and 90.5% in hardness was

observed for single reinforced nanocomposites that were produced

through stir casting and squeeze casting

Tensile strength

The tensile tests were conducted on the test specimens

accord-ing to ASTM E08-8 standards Prior to their loadaccord-ing, the specimens

were first polished with silicon carbide abrasive papers in grit size

ranges from 220 to 800 in order to remove the surface defects on

the sample The universal testing machine (UTM-INSTRON 4000)

loaded with 10 kN load cell was used to conduct the tensile test

The tensile strength was evaluated at the cross head speed of

0.5 mm/min The dimension of the tensile test sample is shown

inFig 6(a) The true stress–strain curves obtained for the

investi-gated single and hybrid reinforced nanocomposites is shown in

Fig 6(b) The variation in ultimate tensile strength (UTS) for the

investigated single and hybrid reinforced nanocomposites is

shown inFig 6(c) When compared to base aluminium alloy, the

nanocomposites were found to possess higher tensile strength

Early researchers proposed different strengthening mechanisms

for composites such as grain refinement, particle strengthening,

load sharing and thermal mismatch strengthening imposed by

nanoparticles Out of these mechanisms, the influence of load

shar-ing effect is minimal[44]and enhancement in tensile strength is

mainly due to grain refinement according to Hall-Petch theory

and the restricted movement of dislocations in the matrix due to

nanoparticles according to Orowan mechanism [45] Increased

strength of nanocomposites could also be attributed to the

differ-ence in CTE of matrix and nanoparticles when it is cooled to room

temperature[46] While these nanocomposites were subjected to

squeeze casting, further grain refinement and porosity reduction

were achieved This might have increased the ultimate tensile

strength of squeeze cast nanocomposites over the stir-cast

nanocomposites In hybrid reinforced nanocomposites, about

8.5% improvement in the ultimate tensile strength was achieved

with the inclusion of a secondary reinforcement (4% SiC) over the

single reinforced nanocomposites, ensuring the uniform

distribu-tion of primary and secondary reinforcements in the matrix About

16.35% higher UTS was observed with the squeeze cast single

rein-forced nanocomposite over the stir cast Lower ductility was

observed with single and hybrid reinforced nanocomposites over

the base alloy and comparatively, it is the lowest in hybrid

forced nanocomposites This might be due to hard ceramic

rein-forcements (Al2O3 and SiC) introduced into the matrix These

reinforcements might have introduced the brittleness and this

lowered the ductility of developed composites However, the

duc-tility behaviour of squeeze cast nanocomposite was superior

among all categories of composites under investigation Under

the influence of squeezing pressure, the space between the den-drites was continuously getting reduced and as a consequence, more fine grains and homogeneous microstructure was obtained with squeeze casting process This is inferred fromFig 8(g) The nucleation rate (N) can be calculated as follows[47]:

where a, b and c are functions of temperature; while p is the squeezing pressure N is getting increased with an increase in p and thus grain refinement is achieved which improves the ductility Thus, in the category of single reinforced nanocomposite, about 31.6% improved ductility was observed with squeeze cast nanocom-posite over stir cast FromFig 9(c), it is evident that ductility can be improved with squeeze casting without any compromise on strength characteristics of nanocomposites

Impact strength The impact strength of single and hybrid reinforced nanocom-posites was determined using Izod impact testing machine according to ASTM E23-07a standards Digital impact testing machine (Fine Testing Machines, Model-FIT - 300 D) was used to determine the impact energy absorbed by the specimens The dimensions of an impact test sample and the impact strength vari-ation for the single and hybrid reinforced nanocomposites are shown inFig 7(a) and (b) respectively When compared to base aluminium alloy, the impact strength of single and hybrid rein-forced nanocomposites were found to be marginally lower This might be due to the fact that the impact strength of a material fol-lows the same trend of ductility However, the squeeze cast nanocomposite was found to have the highest impact strength of all samples The squeeze cast process can reduce the pores and defects to a higher magnitude than stir casting Moreover, it ensures the stronger bond between matrix and reinforcements and grain refinement [48] All these effects collectively result in more ductile material that might have increased the impact strength of the squeeze cast nanocomposite The impact strength

of squeeze cast nanocomposite was 106.3% higher than base alloy that was produced through stir casting

Microstructural examination

Fig 8(a–d) shows the micrographs of AA 7075 base alloy and nanocomposites reinforced with 2 wt.% nano Al2O3particles pro-duced with stir casting at three different reinforcement preheat temperatures 400°C, 500 °C and 600 °C respectively The micro-graph of hybrid reinforced nanocomposites with 2 wt.% and 4 wt

% nano Al2O3 mixed with 4 wt.% SiC content is shown in Fig 8

(e) and (f) More uniform distribution of reinforcements was estab-lished in the hybrid reinforced composite that contained 2 wt.% nano alumina and 4 wt.% SiC particles This is depicted in Fig 8

(e) Keeping the same silicon carbide content, when nano alumina particles were increased from 2 wt.% to 4 wt.% enhanced grain refinement was observed This is shown inFig 8(f) Improved ten-sile strength and hardness as observed in single and hybrid rein-forced nanocomposites can be attributed to grain refinement that was achieved through near uniform distribution of reinforcements

in the matrix The micrograph of a single reinforced nanocomposite developed through squeeze casting is shown inFig 8(g) From this micrograph, it can be inferred that ultra-level grain refinement is possible with squeeze casting than stir casting, even with the same level of nano reinforcement

The scanning electron microscope (SEM) image of aluminium alloy AA7075 (as cast condition) is shown inFig 9(a), while the energy dispersive spectroscopy (EDS) analysis of this alloy is

shownFig 9(b) The SEM image of single reinforced

nanocompos-ite produced through stir casting with 2 wt.% nano Al2O3particles

that were preheated to the temperature of 500°C is shown in

Fig 9(c) The nano Al2O3reinforcements in the base matrix were

identified through the utilisation of higher magnification The

EDS analysis also confirmed the presence of Al2O3nanoparticles

in the matrix This is presented inFig 9(d) It is well proven that

for aluminium metal matrix composites, improved mechanical

properties principally depend upon the uniform distribution of

the second phase in the final composite From SEM images, it

was evident that nanoparticles were almost uniformly distributed

in the base matrix for the composites under investigation It could

be inferred fromFig 9(e), a hybrid reinforced nanocomposite with

2 wt.% nano Al2O3and 4 wt.% micro SiC established the uniform

distribution of reinforcements in the base matrix The presence

of both primary and secondary reinforcement in the base matrix

was confirmed through EDS analysis EDS of the secondary

rein-forcement (silicon carbide) is shown inFig 9(f) While the weight

fraction of primary reinforcement was increased beyond 2%,

agglomeration of both primary and secondary reinforcements

was observed This is shown inFig 9(g) The SEM image of single

reinforced nanocomposite produced by squeeze casting is shown

inFig 9(h) The SEM images of fractured tensile test samples of

2 wt.% Al2O3 reinforced nanocomposite (stir cast), 2 wt.% Al2O3

and 4 wt.% SiC hybrid reinforced nanocomposite (stir cast) and

2 wt.% Al2O3reinforced nanocomposite (squeeze cast) are shown

in Fig 9(i), (j) and (k) respectively The SEM image taken over

the fractured surface of single reinforced squeeze cast

nanocom-posite was exposing some fine dimples and cleavages, which

represented the respective ductile and brittle fracture modes

(referFig 9(k))

Conclusions

This paper addressed the comparative study on mechanical and

microstructural characterisation of AA 7075 based single and

hybrid reinforced nanocomposites produced through stir and

squeeze cast methods with different preheating temperatures

The composites are prepared with reinforcement of 2, 4 wt.% nano

alumina particles and 4 wt.% silicon carbide particles The hybrid

nanocomposite is produced with reinforcing nano alumina and

sil-icon carbide particles The mechanical properties such as density,

porosity, hardness, tensile strength and impact strength are

evalu-ated and compared The significant findings of this investigation

are as follows:

 An increase in hardness and tensile strength is observed for

sin-gle and hybrid reinforced nanocomposites with increasing

Al2O3 content and found to be higher than base aluminium

alloy

 In comparison to base alloy, hardness is getting improved by

63.7% and 81.1% for single and hybrid reinforced

nanocompos-ite (stir cast), while an improvement of 90.5% is observed with

single reinforced nanocomposite (squeeze cast) An increase in

the ultimate tensile strength with magnitudes of 60.1%, 73.8%

and 92.3% is observed with the same sequence of these

compos-ites over the base matrix

 The microstructure and SEM analysis revealed the uniform

dis-tribution of particles in the base matrix provided that the

weight fraction of nano reinforcement is limited to 2%

 Among the different reinforcement preheat temperatures

adopted for fabrication of nanocomposites, 500°C is witnessed

to produce more uniform distribution and prevents

agglomera-tion of particles, while the weight fracagglomera-tion of nano

reinforce-ment is not exceeding 2%

 From the mechanical characterisation tests, it is inferred that the density, hardness and ultimate tensile strength of single and hybrid reinforced nanocomposites are superior to base alloy However, when nano reinforcements are increased beyond 2%, agglomeration of nanoparticle in the base matrix

is inevitable, which deteriorates the mechanical characteristics

of hybrid reinforced nanocomposites

 On the implementation of secondary material processing such

as squeeze casting, even single reinforced nanocomposites own improved properties over hybrid reinforced nanocompos-ites that are produced through stir casting The mechanical and microstructural characterisation of hybrid reinforced nanocomposites by squeeze casting is still to be carried out From this experimental investigation, it is concluded that both squeeze cast single reinforced nanocomposite and stir cast hybrid reinforced nanocomposite exhibit superior mechanical properties over the base alloy, AA 7075 Due to this fact, these composites can be employed as candidate materials in aerospace and automo-tive sectors, where quality is not a compromise

Conflict of interest The authors have declared no conflict of interest

Compliance with Ethics requirements This article does not contain any studies with human or animal subjects

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